Civil rights leader Rev. Jesse Jackson has been diagnosed with Parkinson's disease, he announced Friday.

In an emotional letter to his supporters, the 76-year-old said his diagnosis came "after a battery of tests."

Jackson's father also suffered from the disease.

"For me, a Parkinson's diagnosis is not a stop sign but rather a signal that I must make lifestyle changes and dedicate myself to physical therapy in hopes of slowing the disease’s progression," he wrote.

Parkinson's disease is an uncurable neurodegenerative disorder that affects movement, according to the Mayo Clinic. It can lead to tremors, slowed movement, rigid muscles, loss of movement and speech changes.

Though he had not publicly announced it, Northwestern Medicine said Jackson was diagnosed with the disease in 2015. He has been treated as outpatient in the years since, the hospital said.

Congressman Danny Davis said those who have been close to Jackson "have noticed some of the signs."

Jackson is known for his work as a civil rights activist with Rev. Martin Luther King Jr. and as a Democratic presidential candidate in the 1980s. He founded Chicago's Rainbow PUSH coalition and has remained a prominent religious and political figure, continuing his outspoken activism recently following the fatal shooting of 17-year-old Laquan McDonald in Chicago. He also spoke out about police shootings nationwide, including in Ferguson, Missouri.

"First, my thoughts and prayers are with the family," Mayor Rahm Emanuel said Friday, "but I would note while Parkinson's may be a physical condition, it will never ever break Rev. Jackson's spiritual commitment to justice and his ability to help continue to be a voice for those whose voices are not heard."

Read Jackson's full statement below.

Dear Friends and Supporters,

On July 17, 1960, I was arrested, along with seven other college students, for advocating for the right to use a public library in my hometown of Greenville, S.C. I remember it like it was yesterday, for that day changed my life forever. From that experience, I lost my fear of being jailed for a righteous cause. I went on to meet Dr. King and dedicate my heart and soul to the fight for justice, equality, and equal access. In the tradition of the Apostle Paul, I have offered myself – my mind, body and soul – as a living sacrifice.

Throughout my career of service, God has kept me in the embrace of his loving arms, and protected me and my family from dangers, seen and unseen. Now in the latter years of my life, at 76 years old, I find it increasingly difficult to perform routine tasks, and getting around is more of a challenge. My family and I began to notice changes about three years ago. For a while, I resisted interrupting my work to visit a doctor. But as my daily physical struggles intensified I could no longer ignore the symptoms, so I acquiesced.

After a battery of tests, my physicians identified the issue as Parkinson’s disease, a disease that bested my father.

Recognition of the effects of this disease on me has been painful, and I have been slow to grasp the gravity of it. For me, a Parkinson's diagnosis is not a stop sign but rather a signal that I must make lifestyle changes and dedicate myself to physical therapy in hopes of slowing the disease’s progression.

I am far from alone. God continues to give me new opportunities to serve. This diagnosis is personal but it is more than that. It is an opportunity for me to use my voice to help in finding a cure for a disease that afflicts 7 to 10 million worldwide. Some 60,000 Americans are diagnosed with Parkinson’s every year.

I will continue to try to instill hope in the hopeless, expand our democracy to the disenfranchised and free innocent prisoners around the world. I'm also spending some time working on my memoir so I can share with others the lessons I have learned in my life of public service. I steadfastly affirm that I would rather wear out than rust out.

I want to thank my family and friends who continue to care for me and support me. I will need your prayers and graceful understanding as I undertake this new challenge. As we continue in the struggle for human rights, remember that God will see us through, even in our midnight moments.